Optical encoder system including a structured code wheel

Abstract

An optical encoder system includes a module that has a light emitting element and a light detecting element, wherein the light detecting element is operable to detect light at a wavelength emitted by the light emitting element. The optical encoder system also includes a code wheel that has facets on its surface. Each facet has a surface that provides diffuse reflectance, with adjacent facets being inclined by different amounts. The code wheel can be disposed with respect to the module so that at least some light emitted by the light emitting element is reflected by the facets back toward the module, wherein an amount of reflected light detected by the light detecting element in the module depends at least in part on the rotational position of the code wheel.

What is claimed is: 
     
       1. An optical encoder system comprising
 a module that includes a light emitting element and a light detecting element, wherein the light detecting element is operable to detect light at a wavelength emitted by the light emitting element; and 
 a code wheel including a plurality of facets on its surface, wherein adjacent facets are inclined by different amounts, each facet having a surface that provides diffuse reflectance, and wherein the facets are comprised of a first group and a second group, wherein the facets alternate between the first and second groups in a direction around a central axis of the code wheel, and 
 wherein the code wheel is disposed with respect to the module so that at least some light emitted by the light emitting element is reflected by the facets back toward the module, wherein an amount of reflected light detected by the light detecting element in the module depends at least in part on the rotational position of the code wheel. 
 
     
     
       2. The optical encoder system of  claim 1  wherein adjacent facets are inclined in opposite directions from one another. 
     
     
       3. The optical encoder system of  claim 1  wherein the code wheel has a cylindrical shape and wherein each facet extends along a length direction of the code wheel. 
     
     
       4. The optical encoder system of  claim 1  wherein a surface of each facet is coated with a material that provides diffuse reflectance. 
     
     
       5. The optical encoder system of  claim 4  wherein the material that provides the diffuse reflectance is selected from a group consisting of: polytetrafluoroethylene, barium sulfate, and titanium dioxide. 
     
     
       6. The optical encoder system of  claim 1  wherein the module has optical emission and detection paths that are asymmetric with respect to the code wheel. 
     
     
       7. The optical encoder system of  claim 1 
 wherein when light from the light emitting element is incident on a facet from the first group, the facet reflects light that is detected by the light detecting element at a first intensity level, and 
 wherein when light from the light emitting element is incident on a facet from the second group, the facet reflects light that is detected by the light detecting element at a second different intensity level. 
 
     
     
       8. The optical encoder system of  claim 1  wherein the facets of the first group are coated with a diffusively reflective white material, and the facets of the second group are coated with a diffusively reflective black material. 
     
     
       9. The optical encoder system of  claim 1  wherein each facet has a width in a range of 44-73 μm. 
     
     
       10. An optical encoder system comprising
 a module that includes a light emitting element and a light detecting element, wherein the light detecting element is operable to detect light at a wavelength emitted by the light emitting element; and 
 a code wheel including a plurality of facets on its surface, wherein the code wheel has a cylindrical shape and wherein each facet extends along a length direction of the code wheel, wherein the facets are comprised of facets in a first group and a second group, wherein the facets are disposed around a central axis of the code wheel in an alternating pattern of facets from the first group and the second group, wherein adjacent facets are inclined by different amounts from one another, each facet having a surface that approximates a Lambertian reflective surface, 
 wherein the code wheel is disposed with respect to the module so that light from the module impinges on a facet from the first group at a first angle of incidence, and so that light from the module impinges on a facet from the second group at second angle of incidence shallower than the first angle of incidence. 
 
     
     
       11. The optical encoder system of  claim 10  wherein the surface of each facet is comprised of a material selected from a group consisting of: microstructured polytetrafluoroethylene, microstructured barium sulfate, and microstructured titanium dioxide. 
     
     
       12. The optical encoder system of  claim 10  wherein the surface of each facet in the first group is coated with a first diffusely reflective material and the surface of each facet in the second group is coated with a second diffusely reflective material different from the first diffusely reflective material. 
     
     
       13. The optical encoder system of  claim 10  wherein adjacent facets are inclined in opposite directions from one another. 
     
     
       14. A method of operating an optical encoder system, the method comprising:
 (a) emitting light from an encoder module toward a code wheel, wherein the code wheel has a plurality of facets comprising facets in a first group and a second group, wherein the facets are disposed around a central axis of the code wheel in an alternating pattern of facets from the first group and the second group, wherein adjacent facets are inclined by different amounts from one another; 
 (b) reflecting light from a first facet on the code wheel, wherein the first facet belongs to the first group and has a diffusely reflective surface; 
 (c) detecting in the module a first amount of light reflected by the first facet; 
 (d) emitting more light from the module toward the code wheel; 
 (e) reflecting light from a second facet on the code wheel, wherein the second facet belongs to the second group, is adjacent the first facet, and has a diffusely reflective surface; and 
 (f) detecting in the module a second amount of light reflected by the second facet, 
 wherein (a) through (f) are performed with respect to a first pair of adjacent facets and then for a second pair of adjacent facets. 
 
     
     
       15. The method of  claim 14  wherein emitting light from the encoder module includes emitting infra-red light. 
     
     
       16. The method of  claim 14  wherein the code wheel is disposed with respect to the encoder module so that light from the module impinges on the first facet at a first angle of incidence, and so that light from the module impinges on the second facet at second angle of incidence shallower than the first angle of incidence. 
     
     
       17. The method of  claim 14  wherein the facets of the first group are coated with a diffusively reflective white material, and the facets of the second group are coated with a diffusively reflective black material. 
     
     
       18. The method of  claim 14  the surface of each facet is coated with a material selected from a group consisting of: polytetrafluoroethylene, barium sulfate, and titanium dioxide. 
     
     
       19. The method of  claim 14  wherein each facet has a width in a range of 44-73 μm. 
     
     
       20. The method of  claim 14  wherein light is emitted from the module toward the code wheel along an optical emission path, and light is reflected from the code wheel back toward the module along an optical detection path, wherein the optical emission and detection paths are asymmetric with respect to the code wheel.